1. Field of the Invention
This invention relates generally to a revolution detector of a drive assembly motor of a disk drive, and is more particularly applicable to the revolution detector of a floppy disk drive.
2. Description of the Prior Art
In the so-called disk drive motor of the floppy disk drive, there is provided a revolution detector, such as a speed detector (FG), and is a phase detector (PG). A revolution detector A of this kind by its such that a magnet of the FG is provided in the rotor of the drive motor, and detected by the detector is an electromotive force which is generated in a printed coil of the FG when the coil periodically approaches the magnet due to rotation of the rotor.
Another revolution detector B of this kind is such that a plurality of notches are provided in a side of the shell of the rotor at predetermined intervals, or a plurality of strips which reflect light are stuck on the periphery of the rotor at predetermined intervals, and the light reflected on the notch or the strip is detected by an optical sensor of the revolution detector (see Japanese Laid Open Patent No. 55-122269).
However, the exemplified revolution detector A, the magnet and the coil of the FG become complicated in structure, so that it makes the production of the detector difficult, and increases the production cost. Further, the flux of the magnet of the FG interferes with the flux of the rotary magnet of the rotor, so that the output of the drive motor is reduced.
Moreover, in the revolution detector B using the strips, it is troublesome to stick the strips on the rotor, so that the production cost increases, and further, it is difficult to stick the strips with high accuracy.